As one of the causes of death by traffic accident, there is a sudden change in the physical condition of a driver during driving. A cause of a sudden change in the driver's physical condition includes various diseases such as cerebrovascular diseases and heart diseases. The condition of a driver who cannot continue driving by a sudden change in the physical condition is not constant. Conventionally, there is known a technique for detecting a sudden change in the driver's physical condition (see e.g. Japanese Unexamined Patent Publication No. 2015-021912). In the technique described in Japanese Unexamined Patent Publication No. 2015-021912, a deteriorated physical condition of a driver is estimated based on a driving position of the driver to detect a sign of deterioration of the physical condition.
Generally, it is often the case that by the time when a large change in the driving position of a driver appears. a deteriorated physical condition of the driver progresses to a certain extent. In order to secure safety of a driver, however, it is necessary to detect a deteriorated physical condition of the driver at an early stage before the deteriorated physical condition progresses.